There is known a coupling structure between a piston and a connecting rod using a spherical joint as disclosed in, for example, JP-UM-A-3-17369, JP-A-9-144879, JP-A-2000-337506 and JP-A-2000-213646.
The coupling structure disclosed in JP-UM-A-3-17369 has a semi-spherical concavity 201 formed in the rear surface of the crown of a piston 200, a small end 203 formed on a connecting rod 202 in a substantially spherical shape conforming to the shape of the concavity 201 and two split flanges 204 screwed to the crown surface extending around the concavity 201 of the piston 200 for holding against the piston 200 the small end 203 of the connecting rod 202 fitted slidably in the concavity 201, as shown in FIG. 17.
The coupling structure as described requires for securing the flanges 204 to the piston 200 six screws 206 which add to the number of parts for the structure, its cost and its weight correspondingly. As the number of the screws 206 which are used is large, a correspondingly large amount of time and labor is required for coupling the piston to the connecting rod and thereby brings about a lowering of productivity.
FIG. 18 shows the spherical coupling structure between a piston and a connecting rod as disclosed in JP-A-9-144879. The coupling structure includes a concave spherical washer 212 fitted in a concavity formed in the rear surface of the crown 211 of a piston 210, though not designated, a small end 216 formed on a connecting rod 214 in a spherical shape conforming to a concave spherical surface 213 formed in the concave spherical washer 212, a holder 217 for holding the small end 216 slidably in the concave spherical washer 212, a rotation-preventing member 218 held on the holder 217 and a plurality of pairs of bolts 221 and nuts 222 for securing the concave spherical washer 212, holder 217 and rotation-preventing member 218 together to the piston 210.
The coupling structure as described includes the concave spherical washer 212, holder 217, rotation-preventing member 218 and pairs of bolts 221 and nuts 222 which increase the number of parts for the structure and its weight. The concave spherical washer 212 is, among others, responsible for the increased weight, since it is made of an iron-based material. The increased number of parts as stated requires an increased amount of time and labor for coupling the piston to the connecting rod and thereby brings about a lowering of productivity, as in the case of the coupling structure disclosed in JP-UM-A-3-17369 (FIG. 17).
The coupling structure disclosed in JP-A-2000-337506 includes a small end 235 formed on a connecting rod 234 in a substantially cross shape having a spherical surface, as shown in FIG. 19A. The substantially cross-shaped small end 235 has a first protrusion 236 and a second protrusion 237 crossing each other at right angles, the second protrusion 237 being smaller in diameter than the first protrusion 236. A piston 230 has a concavity 241 (FIG. 19B) formed in the rear surface of a crown 231 and having a spherical inner wall surface. The concavity 241 has at its open end a cross-shaped opening 242 conforming to the shape of the small end 235 of the connecting rod 234. For assembly, the protrusion 237 of the connecting rod 234 is inserted into the concavity 241 of the piston 230 through its cross-shaped opening 242 and turned by 90 degrees, whereby the piston 230 and the connecting rod 234 are coupled together.
The coupling structure as described makes it possible to reduce the number of parts and the amount of time and labor as required for assembly, since it does not require any of the screws, bolts or other fasteners as shown in FIGS. 17 and 18. However, the opening 242 of the concavity 241 of the piston 230 and the small end 235 of the connecting rod 234, which are both cross-shaped, require a complicated machining job and need to be improved from the standpoints of productivity and production cost. The cross-shaped small end 235 of the connecting rod 234 has a relatively small area of contact with the wall of the concavity 241 of the piston 230, which increases the pressure acting upon the joint and thereby lowers its coupling strength.
FIG. 20 shows the spherical coupling structure between a piston and a connecting rod as disclosed in JP-A-2000-213646. The coupling structure has a semi-spherical concavity 251 formed in the rear surface of the crown of a piston 250, a fixing plate 252 secured to the rear surface of the crown of the piston 250 and a spherical small end 256 of a connecting rod 254 held slidably in a spherical space defined by the concavity 251 of the piston 250 and a spherical inner surface 253 of the fixing plate 252. The small end 256 of the connecting rod 254 has its center C (i.e. the center of the joint between the piston 250 and the connecting rod 254) offset radially of the piston 250 by an amount δ of eccentricity from its axial centerline CL.
The offsetting of the center C of the joint between the piston 250 and the connecting rod 254 by δ from the axial centerline of the piston 250 prevents the piston 250 from rotating about the center C of its joint with the connecting rod 254 by causing the piston 250 to contact the wall surface of a cylinder. When the piston 250 and the cylinder has a large clearance therebetween during, for example, the beginning of engine operation when the piston 250 is still at a low temperature, however, the piston 250 rotates about the connecting rod 254 and if the piston 250 contacts the cylinder in any portion of high rigidity other than its skirts, the piston 250 gives the cylinder a large striking force which is likely to deprive the clearance between the piston 250 and the cylinder of a lubricant oil film. It is, therefore, desirable to be able to prevent the rotation of the piston 250 about the connecting rod 254 irrespective of the stage of engine operation. It is also desirable to prevent the rotation of the piston 250 without incurring any additional cost.
Therefore, it is desirable to provide a spherical coupling structure between a piston and a connecting rod which requires only a small number of parts, is thus inexpensive and small in weight and thereby improves productivity, and which makes it possible to prevent the rotation of the piston about the connecting rod irrespective of the stage of operation of an internal combustion engine and without incurring any additional cost.